📚 Respiration and Circulation: How the Body Uses Oxygen | 呼吸与循环：人体如何利用氧气
Respiration is the chemical process that releases energy from food in every living cell. This chapter explains how oxygen reaches body cells through gas exchange and circulation, and why this energy transfer is essential for life.
呼吸作用是每个活细胞中从食物释放能量的化学过程。本章解释氧气如何通过气体交换和循环到达身体细胞，以及为什么这种能量转移对生命至关重要。
1. What is Respiration? | 什么是呼吸作用？
In biology, respiration is not the same as breathing. Respiration is a chemical reaction that takes place inside cells. It releases energy from glucose so that cells can carry out their functions.
在生物学中，呼吸作用与呼吸不同。呼吸作用是发生在细胞内部的一种化学反应。它从葡萄糖中释放能量，使细胞能够执行其功能。
All living organisms need energy to stay alive. In humans and many other organisms, this energy is usually released by aerobic respiration when oxygen is available.
所有生物都需要能量来维持生命。在人类和许多其他生物中，当氧气充足时，这种能量通常通过有氧呼吸作用释放。
2. Aerobic Respiration and Its Word Equation | 有氧呼吸及其文字方程式
The word equation for aerobic respiration is:
有氧呼吸的文字方程式是：
glucose + oxygen → carbon dioxide + water + energy
葡萄糖 + 氧气 → 二氧化碳 + 水 + 能量
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)
Aerobic means 'with oxygen'. The reaction is exothermic: it transfers energy to the surroundings of the cell, not just as heat but as chemical energy the cell can use.
有氧的意思是“需要氧气”。这是一个放热反应：它将能量转移给细胞周围环境，不仅以热量形式，还以细胞可用的化学能形式存在。
3. Why Living Cells Need Energy | 活细胞为什么需要能量
Energy is needed for muscle contraction, active transport, cell division, growth, and maintaining a constant body temperature.
能量用于肌肉收缩、主动运输、细胞分裂、生长以及维持恒定体温。
- Movement — muscle cells need energy to contract and relax. 运动——肌肉细胞需要能量来收缩和舒张。
- Active transport — cells move substances across membranes against a concentration gradient. 主动运输——细胞逆浓度梯度跨膜运输物质。
- Cell division and growth — new cells are produced and damaged tissues are repaired. 细胞分裂与生长——产生新细胞并修复受损组织。
- Keeping warm — in mammals, respiration releases heat to help maintain body temperature at about 37 °C. 保暖——在哺乳动物中，呼吸作用释放热量以帮助维持约 37 °C 的体温。
4. The Human Gas Exchange System | 人体气体交换系统
The human gas exchange system includes the nose, trachea, bronchi, bronchioles, and alveoli. Its role is to bring oxygen from the air into the blood and remove carbon dioxide from the blood into the air.
人体气体交换系统包括鼻、气管、支气管、细支气管和肺泡。它的作用是将空气中的氧气带入血液，并将血液中的二氧化碳排出到空气中。
Air travels from the nose or mouth through the trachea, which branches into two bronchi. Each bronchus divides many times into smaller bronchioles. At the end of the bronchioles are millions of tiny air sacs called alveoli.
空气从鼻或口进入，经过气管，气管分为两根支气管。每根支气管又多次分支成更小的细支气管。细支气管末端是数百万个微小气囊，称为肺泡。
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